This invention relates to a method of stacking seismic data to identify a fault in a geological seam. The invention is particularly, but not exclusively, applicable to locating faults in coal seams.
The advance longwall face method of mining which is typically used in modern coal mines is highly capital intensive. It takes several months to set up a coal face and capital costs are high. A significant proportion of the longwall faces encounter small but serious geological faults. These, almost invariably, disrupt production. In a substantial number of instances, unexpected faulting results in the premature abandonment of faces. Faults usually affect the integrity of a hydraulically supported roof. Faults encountered head-on bring consequential flooding and/or fire risks. Thus, unanticipated faulting can seriously affect the economics of mining and it is therefore highly desirable to know the geological structure of a seam prior to mining.
A number of techniques are available for investigating subsurface geology. Direct methods include borehole drilling, both vertically from the surface, and horizontally from the coal face itself. Indirect or geophysical methods include seismology, resistivity, gravity and borehole geophysical techniques. Of these, seismology, as practiced from the surface of the earth, tells most about the subsurface structure. Large faults may be detected; however, small but vitally important faults can be missed. For this reason seismology is now also applied underground since it is known that a coal seam will guide, or channel seismic waves.
In-seam seismology differs from standard surface seismology in two main respects. Firstly, channel waves are dispersive. The signal from a quasi-impulsive source, e.g. a small explosion, is gradually phase encoded as it propagates. The initial sharp signal spreads out as it travels through the coal. Secondly, the underground environment is harsh. There are very stringent safety regulations in most countries governing the use of equipment under-ground and movement is restricted.
To obtain a viable production system the underground field technique must therefore be made as simple as possible.
The known method of in-seam seismology is to locate geophones in short, horizontal boreholes in the coal seam. Small charges are fired consecutively from other boreholes in the seam. The signals received by the geophones are recorded using a standard seismic recording system with say, 12 channels and a sampling rate of 0.5 ms.
The problem of dispersion can be handled using specialised data processing techniques. As shown in our copending Patent Application No. 86,628, dispersive seismic arrivals can be recompressed to appear as impulse-like events.
Reference has been made above to the standard surface seismology which is used. The information gained by such seismology is subjected to processing by a technique known as Common Depth Point (CDP) stacking. This is a method designed to improve the signal to noise ratio in seismograms and highlights seismic reflectors. CDP stacking is greatly facilitated by using a special field recording technique with predetermined locations for shots and geophones. The improvement in signal to noise ratio arises because the totality of seismic traces is processed to produce a smaller number of improved traces. There is in fact data redundancy because reflection points in the earth may be sampled more than once.
In underground seismology the environment does not permit the luxury of data redundancy. The field technique must be made simple.
Unfortunately, the term `common depth point` is inaccurate. Only in the case of a horizontal reflector is the term accurate. For a dipping reflector, the term is a misnomer. In such cases CDP stacking puts the reflectors in the wrong spatial positions. It is necessary to migrate the data to correct for this effect. Migration procedures, such as wave equation migration, require large quantities of data if they are to be accurate. In channel wave seismology large quantities of data are not available, and reflectors may occur at any angle of `dip` to the geophone line. For large dips the standard midpoint gathers are inappropriate.
In surface seismology dispersion of seismic waves is usually neglected, but for in-seam seismology it is vitally important. In co-pending British Patent Application No. 41420/78 it is shown how channel wave data may be recompressed to produce impulse-like events but even when this is done the group and phase velocities remain different. Standard CDP methods do not differentiate between these two velocities.